Process for absorption of diamondoids from natural gas

ABSTRACT

A process for removing diamondoids from a stream of natural gas. In this process, contact between a solvent liquid and natural gas containing diamondoids occurs in a stepwise counter current cascaded fashion. The stepwise counter-current cascaded arrangement substantially improves the removal of the light diamondoids (adamantane). The contacting (mixing) of the gas and solvent liquid allows the liquid to absorb the diamondoids contained in the gas. The gas and liquid are then separated thereby removing the diamondoids from the gas. In the preferred embodiment, a multi -staged trayed solvent contractor tower is used to facilitate the stepwise counter current cascaded contacting in the gas production process immediately after the separation of the gas from any liquid in the gas. The partially saturated diamondoid liquid solvent from the tower can also be mixed with the gas at a point before the separation of the gas from any reservoir formation liquid in order to enhance the diamondoid removal process.

FIELD OF THE INVENTION

This invention relates to the removal of diamondoid components fromnatural gas streams during gas processing. In particular, it relates toabsorbing the diamondoid compounds into a liquid solvent that contactsthe natural gas.

BACKGROUND OF THE INVENTION

Certain natural gas streams contain diamondoid compounds. Thesediamondoid compounds are a family of volatile C10+ hydrocarbons withlattice-like structures. Some of these diamondoids form solids at warmerthan ambient (greater than 100 ° F.) temperatures when they arecondensed from natural gas streams. These diamondoids are present intrace quantities in the natural gas reservoirs themselves.

New sources of hydrocarbon gases are now being produced which containsignificantly higher concentrations of diamondoids. Whereas in the pastthe amount of diamondoid compounds has typically been too small to causeoperational problems such as plugging of production equipment, these newsources have experienced the problem of plugging the natural gasproduction equipment. Costly maintenance and repair time is required toremove the diamondoids. These diamondoid solids must be removed from thenatural gas stream by appropriate means to maintain satisfactoryproduction operations.

Although recent patents by Alexander et al. (U.S. Pat. Nos. 4,952,747;4,952,748; 4,952,749; 4,982,049) have disclosed methods of removing somefractions of diamondoids from gas streams, the adamantane fraction ofdiamondoids in particular produced with the natural gas cannot beadequately absorbed by the methods disclosed in the Alexander patentsfor diamondoid removal (such as contacting the gas stream with a liquidsolvent in which diamondoids are partially soluble, by injecting thesolvent into producing wellheads and flowlines and subsequently removingthe diamondoids by separating the gas from the diamondoid-ladensolvent). Although diamondoid content is reduced, these methods do notremove a sufficient portion of the diamondoids to allow satisfactoryoperation of equipment without the concern of diamondoids pluggingdownstream equipment. In fact, additional processes such as silica gelabsorbents as described in U.S. Pat. No. 4,952,748 may be required forfurther diamondoid removal to satisfactory levels.

With the current methods, a sufficient amount of diamondoids may beremoved only if an extremely large amount of liquid solvent is mixedwith the diamondoid-laden gas. Therefore, problems associated withresidual amounts of the light diamondoids still remain and can occurwhen using the current methods in several instances such as thefollowing: 1) diamondoid amounts absorbed by downstream glycoldehydration solvent (e.g. 0.06 lb/gal of glycol) would quickly load theglycol system charcoal filters. Frequent (weekly) filter change-outwould be necessary to maintain the capability of removing diamondoids aswell as other normal glycol degradation products in order to preventglycol fouling and foaming problems; 2) flash gas recompressionequipment, which returns low pressure gas from the glycol dehydrationregeneration systems to the main gas flow would be handling gas streamswith significant (up to 1500 lb/MSCF) diamondoid concentrations. Uponcompression and cooling, diamondoids in this flash gas would precipitateand likely plug compressor discharge coolers and inlet scrubber mistextractors; and 3) diamondoids remaining in the dehydrated gas mayprecipitate in the produced gas pipeline, thereby potentially causingscaling and plugging problems. Therefore, there remains a need for adiamondoid removal process that can remove a sufficient amount ofdiamondoids from the natural gas, such that equipment problems caused byresidual amounts of diamondoids are reduced, without the use ofunusually large amounts of solvent.

SUMMARY OF THE INVENTION

The present invention provides a method for removal of diamondoidcompounds from a natural gas stream by using a liquid solvent to absorbthe diamondoids. This method of removing the diamondoid compounds fromthe natural gas can be implemented by contacting the diamondoid ladennatural gas with a lean (diamondoid free) liquid solvent in acounter-current fashion such that the lean liquid initially contacts theleanest diamondoid gas (at the top of the tower) and then the liquidflows down the tower countercurrently through the diamondoid gas flowwhile progressively absorbing diamondoids from the richer diamondoid gaswhich is flowing up from the lower sections of the tower. This methodcan be implemented by initially separating liquid and natural gas from awellhead into separate phases before contacting the natural gas in theliquid solvent absorber tower. This initial separation removes somediamondoids from the natural gas. The gas then flows to the absorbertower where more diamondoids are removed. The now diamondoid lean qas isseparated from the now rich liquid solvent by the absorber tower itself.This partially saturated liquid solvent can then be injected into thewellhead flowline at a point before the initial separation of the liquidand natural gas such that it can mix with the diamondoid-laden gas andthereby absorb more diamondoids. This reduces diamondoid concentrationin the natural gas flowing into the aforementioned contactor therebyenhancing efficiency of the process. In the implementation of thisinvention, a multi-staged trayed solvent contractor tower can be used tofacilitate the actual contacting of the liquid solvent with the naturalgas and the absorption of the diamondoids into the liquid solvent.Trayed towers and other similar devices bring about contact between thelean liquid solvent and diamondoid rich gas. The contact between liquidsolvent and the gas causes the molecular mass transfer of thediamondoids to occur such that the diamondoid content of the gas isdecreased and the diamondoid content of the liquid is converselyincreased.

The method of this invention does not completely remove all of thediamondoids from the natural gas; however, this method will remove asufficient amount of diamondoids from the natural gas stream such thatthe aforementioned problems would be manageable. The trace amounts ofprecipitating diamondoids in the natural gas and producing gas streamswill be low enough that their accumulation will be inconsequential tothe operation of the equipment

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1. Simplified flow schematic of the present invention using asolvent wash tower.

FIG. 2A. Cross-section of Solvent Contacting Tower.

FIG. 2B. Detail of Gas and Liquid Contacting Tray in Wash Tower.

FIG. 3. Current process for removing diamondoids by mixing solvent withthe natural gas containing diamondoids.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 refers to a simplified flow schematic of the preferred embodimentof this invention for removing diamondoid compounds from natural gas.Diamondoid rich gas along with liquids from the reservoir formation atthe wellhead 1 are mixed with a partially saturated liquid solvent at apoint 12 before the gas enters the separator 2. At this stage the liquidsolvent absorbs some of the diamondoids from the natural gas. Theproduction separator 2 separates the natural gas from the liquid anddischarges them through separate lines 3 and 4. The separation of thenatural gas from the liquid removes the diamondoids absorbed thus farfrom the natural gas. The discharged natural gas then enters the solventcontactor tower 5. The contactor can have multi-staged trays, as shownin FIGS. 2A and 2B; however, other similar devices such as packed towerscan perform the same functions as the contactor tower. FIG. 2B gives anexample of typical contactor mixing trays. The operation of thiscontactor tower is already known to those skilled in the art.

In the preferred embodiment, diamondoid rich natural gas enters thetower 5 at a point near the bottom of the tower from line 3 and flowsupward. A liquid solvent, such as diesel, enters the tower near the topfrom line 8 and flows downward. The tower FIG. 2A facilitates stepwisecontact of liquid and gas in a counter current cascaded fashion. On eachtray 15 of the tower, for example, the gas and liquid are brought intointimate contact (mixed) and then separated. This stepwise contactoccurs because of the opposite flow of the natural gas and the liquidsolvent in the tower. The contact and mixing of the gas and liquidallows chemical equilibrium forces to cause the molecular mass transferof diamondoids to occur such that the diamondoid content of the gas isdecreased and the diamondoid content of the liquid is converselyincreased.

In order to maintain the maximum chemical equilibrium force, the tower 5orientation is such that the leanest diamondoid gas is initiallycontacted by the leanest diamondoid liquid (at the top of the tower) andthe richest diamondoid gas is contacted by the richest diamondoid liquid(at the bottom of the tower). The gas becomes progressively leaner indiamondoids as it travels up the tower length due to the cumulativeabsorption of diamondoids from the gas at each tray. Conversely, theliquid becomes progressively richer in diamondoids as it travels downthe tower length due to the cumulative absorption of diamondoids intothe liquid at each tray.

The diamondoid lean natural gas is discharged from the contactor tower5, and flows through line 6 to other processing equipment such as aglycol dehydration unit 9. The glycol dehydration unit 9 removes waterfrom the gas before pipeline transport.

Because diamondoids will quickly load up the charcoal filters in theglycol dehydration circulation system, this solvent wash contractorprocess must remove a substantial portion of the diamondoids(particularly adamantane fractions) from the natural gas stream beforethe gas reaches the glycol dehydration process. In order to achieveadequate diamondoid removal, prior art processes require largequantities of 100% fresh diamondoid solvent or additional downstreamprocesses such as silica gel. However, the embodiment shown in FIG. 1uses less fresh liquid solvent because this process allows the solventfrom the contactor to be used a second time by pumping the solvent intothe upstream end of each flowline at a point 12 before the gas entersthe production separator 2. Mixing this solvent with the wellhead gascan remove some diamondoids from the gas and can prevent diamondprecipitation problems at the wellhead facilities immediately downstreamof the wellhead. The use of the absorber contactor with fresh solventand reinjection of the contactor's discharged solvent not only increasesthe diamondoid removal but also reduces the solvent circulation ratesfrom approximately 6.7 BL/MSCF to approximately 2.7 BBL/MSCF for thepresent invention as shown in Table 1.

As stated above one advantage of the present invention is that a largerpercentage of diamondoids can be removed from the natural gas, by usingless liquid solvent. The cleaner natural gas decreases the likelihood ofequipment failure due to diamondoid contamination, thereby decreasingthe amount of equipment maintenance time. Comparisons between thepresent method of wellhead injection only (Case 1) and this invention(Case 2) with a tower and wellhead injection are shown in Table 1 below.

Case 1 is illustrated in FIG. 3. This case involves mixing the naturalgas at the wellhead with a fresh diesel solvent, then separating the gasand the solvent. As stated previously, this method has a substantiallyhigher diesel circulation rate than the present invention. This methodremoves some (approximately 95%) of the diamondoids from the gas. Also,as shown in Table 1, if the gas flowed through a glycol contactor 11lb/hour of diamondoids would be absorbed by the glycol contactor.

Case 2 is the preferred embodiment of this invention and is illustratedin FIG. 1. As described previously, natural gas is initially mixed witha diesel solvent which has been recycled from the wash tower 5. Thissolvent contains some diamondoids from prior contact with natural gas inthe wash tower. The solvent absorbs some of the diamondoids from the gaswhile in the flowline. The gas solvent mixture is then separated in theseparator 2. The gas then enters the wash tower and contacts freshdiesel, where substantially all of the remaining diamondoids are removedfrom the gas. If the gas then flowed through a Glycol contactor only 1lb/hour of diamondoids would be absorbed by the Glycol contactor. Thisnumber is substantially lower than simple flowline injection and uses asubstantially lower amount of diesel.

While several embodiments and types of equipment have been described andillustrated, it will be understood that the invention is not limitedthereto, since modifications may be made and will become apparent tothose skilled in the art.

We claim that:
 1. A process for removing diamondoid compounds fromnatural gas comprising the steps of:a) providing a natural gas streamcontaining a concentration of diamondoid compounds; b) mixing thenatural gas with a liquid diamondoid absorbing solvent in a cascadingmanner where diamondoid rich natural gas contacts the lean solvent sothat the solvent absorbs the diamondoids from the natural gas; and c)separating the diamondoid rich liquid solvent from the diamondoid leannatural gas.
 2. A process of claim 1 wherein the mixing step (b) and theseparating step (c) occur in a multi-staged solvent wash tower.
 3. Aprocess of claim 1 wherein the diamondoid absorbing solvent is comprisedof diesel fluid.
 4. A process for removing diamondoid compounds from anatural gas stream during natural gas processing comprising the stepsof:a) providing a natural gas stream containing a concentration ofdiamondoid compounds; b) separating any liquid mixed with the naturalgas so that the liquid and natural gas and liquid components areprocessed separately; c) mixing the natural gas with a liquid diamondoidabsorbing solvent in a cascading manner where diamondoid rich naturalgas contacts the diamondoid lean solvent so that the solvent absorbsdiamondoids from the natural gas; and d) separating the diamondoid richliquid solvent from the diamondoid lean natural gas.
 5. A process ofclaim 4 wherein the mixing step (c) and separating step (d) occur in amulti-staged solvent wash tower.
 6. A process of claim 4 wherein thediamondoid absorbing solvent is comprised of diesel fluid.
 7. A processof claim 4 further comprising the step of recycling the partiallysaturated solvent of step (d) back into the flowline at a point in theflowline before step (b) so that the liquid solvent absorbs some of thediamondoids.
 8. A process for removing diamondoid compounds from anatural gas stream comprising the steps of:a) providing a natural gasstream in a flowline; b) mixing a liquid solvent that is partiallysaturated with diamondoids with the natural gas in the flowline so thatthe liquid solvent absorbs some of the diamondoids out of the gas; c)separating the liquid solvent from the gas; d) mixing the natural gaswith a fresh liquid diamondoid absorbing solvent in a cascading mannerwhere diamondoid rich gas contacts diamondoid lean solvent so that thesolvent absorbs diamondoids from the natural gas; and e) separating thesolvent from the diamondoid lean natural gas.
 9. A process of claim s,wherein the mixing step (d) and the separating step (e) occur in amulti-staged solvent wash tower.
 10. A process of claim 8, wherein thediamondoid absorbing solvent is comprised of diesel fluid.
 11. A processof claim 8 further comprising the step of recycling the partiallysaturated solvent of step (e) back into the flowline at a point in theflowline such that the mixing step (b) will occur.